1zram: Compressed RAM based block devices 2---------------------------------------- 3 4* Introduction 5 6The zram module creates RAM based block devices named /dev/zram<id> 7(<id> = 0, 1, ...). Pages written to these disks are compressed and stored 8in memory itself. These disks allow very fast I/O and compression provides 9good amounts of memory savings. Some of the usecases include /tmp storage, 10use as swap disks, various caches under /var and maybe many more :) 11 12Statistics for individual zram devices are exported through sysfs nodes at 13/sys/block/zram<id>/ 14 15* Usage 16 17There are several ways to configure and manage zram device(-s): 18a) using zram and zram_control sysfs attributes 19b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org). 20 21In this document we will describe only 'manual' zram configuration steps, 22IOW, zram and zram_control sysfs attributes. 23 24In order to get a better idea about zramctl please consult util-linux 25documentation, zramctl man-page or `zramctl --help'. Please be informed 26that zram maintainers do not develop/maintain util-linux or zramctl, should 27you have any questions please contact util-linux@vger.kernel.org 28 29Following shows a typical sequence of steps for using zram. 30 31WARNING 32======= 33For the sake of simplicity we skip error checking parts in most of the 34examples below. However, it is your sole responsibility to handle errors. 35 36zram sysfs attributes always return negative values in case of errors. 37The list of possible return codes: 38-EBUSY -- an attempt to modify an attribute that cannot be changed once 39the device has been initialised. Please reset device first; 40-ENOMEM -- zram was not able to allocate enough memory to fulfil your 41needs; 42-EINVAL -- invalid input has been provided. 43 44If you use 'echo', the returned value that is changed by 'echo' utility, 45and, in general case, something like: 46 47 echo 3 > /sys/block/zram0/max_comp_streams 48 if [ $? -ne 0 ]; 49 handle_error 50 fi 51 52should suffice. 53 541) Load Module: 55 modprobe zram num_devices=4 56 This creates 4 devices: /dev/zram{0,1,2,3} 57 58num_devices parameter is optional and tells zram how many devices should be 59pre-created. Default: 1. 60 612) Set max number of compression streams 62Regardless the value passed to this attribute, ZRAM will always 63allocate multiple compression streams - one per online CPUs - thus 64allowing several concurrent compression operations. The number of 65allocated compression streams goes down when some of the CPUs 66become offline. There is no single-compression-stream mode anymore, 67unless you are running a UP system or has only 1 CPU online. 68 69To find out how many streams are currently available: 70 cat /sys/block/zram0/max_comp_streams 71 723) Select compression algorithm 73Using comp_algorithm device attribute one can see available and 74currently selected (shown in square brackets) compression algorithms, 75change selected compression algorithm (once the device is initialised 76there is no way to change compression algorithm). 77 78Examples: 79 #show supported compression algorithms 80 cat /sys/block/zram0/comp_algorithm 81 lzo [lz4] 82 83 #select lzo compression algorithm 84 echo lzo > /sys/block/zram0/comp_algorithm 85 86For the time being, the `comp_algorithm' content does not necessarily 87show every compression algorithm supported by the kernel. We keep this 88list primarily to simplify device configuration and one can configure 89a new device with a compression algorithm that is not listed in 90`comp_algorithm'. The thing is that, internally, ZRAM uses Crypto API 91and, if some of the algorithms were built as modules, it's impossible 92to list all of them using, for instance, /proc/crypto or any other 93method. This, however, has an advantage of permitting the usage of 94custom crypto compression modules (implementing S/W or H/W compression). 95 964) Set Disksize 97Set disk size by writing the value to sysfs node 'disksize'. 98The value can be either in bytes or you can use mem suffixes. 99Examples: 100 # Initialize /dev/zram0 with 50MB disksize 101 echo $((50*1024*1024)) > /sys/block/zram0/disksize 102 103 # Using mem suffixes 104 echo 256K > /sys/block/zram0/disksize 105 echo 512M > /sys/block/zram0/disksize 106 echo 1G > /sys/block/zram0/disksize 107 108Note: 109There is little point creating a zram of greater than twice the size of memory 110since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the 111size of the disk when not in use so a huge zram is wasteful. 112 1135) Set memory limit: Optional 114Set memory limit by writing the value to sysfs node 'mem_limit'. 115The value can be either in bytes or you can use mem suffixes. 116In addition, you could change the value in runtime. 117Examples: 118 # limit /dev/zram0 with 50MB memory 119 echo $((50*1024*1024)) > /sys/block/zram0/mem_limit 120 121 # Using mem suffixes 122 echo 256K > /sys/block/zram0/mem_limit 123 echo 512M > /sys/block/zram0/mem_limit 124 echo 1G > /sys/block/zram0/mem_limit 125 126 # To disable memory limit 127 echo 0 > /sys/block/zram0/mem_limit 128 1296) Activate: 130 mkswap /dev/zram0 131 swapon /dev/zram0 132 133 mkfs.ext4 /dev/zram1 134 mount /dev/zram1 /tmp 135 1367) Add/remove zram devices 137 138zram provides a control interface, which enables dynamic (on-demand) device 139addition and removal. 140 141In order to add a new /dev/zramX device, perform read operation on hot_add 142attribute. This will return either new device's device id (meaning that you 143can use /dev/zram<id>) or error code. 144 145Example: 146 cat /sys/class/zram-control/hot_add 147 1 148 149To remove the existing /dev/zramX device (where X is a device id) 150execute 151 echo X > /sys/class/zram-control/hot_remove 152 1538) Stats: 154Per-device statistics are exported as various nodes under /sys/block/zram<id>/ 155 156A brief description of exported device attributes. For more details please 157read Documentation/ABI/testing/sysfs-block-zram. 158 159Name access description 160---- ------ ----------- 161disksize RW show and set the device's disk size 162initstate RO shows the initialization state of the device 163reset WO trigger device reset 164mem_used_max WO reset the `mem_used_max' counter (see later) 165mem_limit WO specifies the maximum amount of memory ZRAM can use 166 to store the compressed data 167max_comp_streams RW the number of possible concurrent compress operations 168comp_algorithm RW show and change the compression algorithm 169compact WO trigger memory compaction 170debug_stat RO this file is used for zram debugging purposes 171backing_dev RW set up backend storage for zram to write out 172 173 174User space is advised to use the following files to read the device statistics. 175 176File /sys/block/zram<id>/stat 177 178Represents block layer statistics. Read Documentation/block/stat.txt for 179details. 180 181File /sys/block/zram<id>/io_stat 182 183The stat file represents device's I/O statistics not accounted by block 184layer and, thus, not available in zram<id>/stat file. It consists of a 185single line of text and contains the following stats separated by 186whitespace: 187 failed_reads the number of failed reads 188 failed_writes the number of failed writes 189 invalid_io the number of non-page-size-aligned I/O requests 190 notify_free Depending on device usage scenario it may account 191 a) the number of pages freed because of swap slot free 192 notifications or b) the number of pages freed because of 193 REQ_DISCARD requests sent by bio. The former ones are 194 sent to a swap block device when a swap slot is freed, 195 which implies that this disk is being used as a swap disk. 196 The latter ones are sent by filesystem mounted with 197 discard option, whenever some data blocks are getting 198 discarded. 199 200File /sys/block/zram<id>/mm_stat 201 202The stat file represents device's mm statistics. It consists of a single 203line of text and contains the following stats separated by whitespace: 204 orig_data_size uncompressed size of data stored in this disk. 205 This excludes same-element-filled pages (same_pages) since 206 no memory is allocated for them. 207 Unit: bytes 208 compr_data_size compressed size of data stored in this disk 209 mem_used_total the amount of memory allocated for this disk. This 210 includes allocator fragmentation and metadata overhead, 211 allocated for this disk. So, allocator space efficiency 212 can be calculated using compr_data_size and this statistic. 213 Unit: bytes 214 mem_limit the maximum amount of memory ZRAM can use to store 215 the compressed data 216 mem_used_max the maximum amount of memory zram have consumed to 217 store the data 218 same_pages the number of same element filled pages written to this disk. 219 No memory is allocated for such pages. 220 pages_compacted the number of pages freed during compaction 221 huge_pages the number of incompressible pages 222 2239) Deactivate: 224 swapoff /dev/zram0 225 umount /dev/zram1 226 22710) Reset: 228 Write any positive value to 'reset' sysfs node 229 echo 1 > /sys/block/zram0/reset 230 echo 1 > /sys/block/zram1/reset 231 232 This frees all the memory allocated for the given device and 233 resets the disksize to zero. You must set the disksize again 234 before reusing the device. 235 236* Optional Feature 237 238= writeback 239 240With incompressible pages, there is no memory saving with zram. 241Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page 242to backing storage rather than keeping it in memory. 243User should set up backing device via /sys/block/zramX/backing_dev 244before disksize setting. 245 246= memory tracking 247 248With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the 249zram block. It could be useful to catch cold or incompressible 250pages of the process with*pagemap. 251If you enable the feature, you could see block state via 252/sys/kernel/debug/zram/zram0/block_state". The output is as follows, 253 254 300 75.033841 .wh 255 301 63.806904 s.. 256 302 63.806919 ..h 257 258First column is zram's block index. 259Second column is access time since the system was booted 260Third column is state of the block. 261(s: same page 262w: written page to backing store 263h: huge page) 264 265First line of above example says 300th block is accessed at 75.033841sec 266and the block's state is huge so it is written back to the backing 267storage. It's a debugging feature so anyone shouldn't rely on it to work 268properly. 269 270Nitin Gupta 271ngupta@vflare.org 272
